Digital Subscriber Line (DSL) is a common term for various techniques, which make it possible to transmit digital traffic over ordinary telephone lines. Thus, a normally used abbreviation is xDSL wherein x can be replaced by a letter that identifies the technique in question. So the acronym xDSL refers collectively to a number of variations of the DSL technology, which aim at utilizing the information transmission capability of ordinary copper wires to the greatest possible extent. Known variations that go under the umbrella definition of xDSL are at the priority date of this patent application ADSL (Asymmetric Digital Subscriber Line), CDSL (Consumer DSL, registered trademark of Rockwell International Corp.), G. Lite (also known as DSL Lite, splitterless ADSL, and Universal ADSL; officially ITU-T standard G-992.2), HDSL (High bit-rate DSL), RADSL (Rate-Adaptive DSL), SDSL (Symmetric DSL), SHDSL (Symmetric High bit-rate DSL.), VDSL (Very high data rate DSL) and even to some extent UDSL (Unidirectional DSL), which is only a proposal, and IDSL (ISDN DSL), which is actually closer to ISDN (Integrated Services Digital Network).
As mentioned, the DSL technology provides transport of digital information over telephone subscriber lines. High speed digital transmission via telephone lines requires advanced signal processing to overcome transmission impairments due to crosstalk from the signals present on other wires in the same cable, and signal reflections. FIG. 1 illustrates an example of the crosstalk caused by the other lines. The central office (CO) 1 transmits line specific signals to a number of customer premises equipment 2. Each line uses an individual transmission power. As illustrated in FIG. 1, the first signal to the first CPE 2A is exposed to the crosstalk from the other lines, which can be detected at the receiving end. The crosstalk of the single line to the first line is illustrated as a dashed line. This type of crosstalk is called far-end crosstalk (FEXT), wherein crosstalk noise from a transmitted signal is detected at the receiving end of a parallel line. Naturally, all receiving ends are similarly exposed to crosstalk from parallel lines.
Since each line affects parallel lines and transmission times vary from line to line, a great number of situations exist where crosstalk conditions are different. When the transmission power of a line is high, it produces a good signal-to-noise ratio (SNR), but at the same time it also increases crosstalk in parallel lines.
At present, the transmission power in digital subscriber lines is constant with an option for power reduction (backoff) in certain situations. The optional power backoff is based on only the measured signal attenuation in the line.
According to the known solutions, nominal transmission power generates unnecessary heat. The heat exhausts and even damages the components and devices of a subscriber line. Further, the known solutions induce unnecessarily high crosstalk situations, which may even lead to link failures. Furthermore, the known solutions do not utilize the transmission capacity of subscriber lines effectively.
The goal of the invention is to alleviate the above-mentioned drawbacks of the known solutions. The goal is achieved in a way described in the claims.